The generation of heat is an inherent consequence of the controlling of electric power flow by components of a circuit board. Effective dissipation of such heat permits the circuit board to carry more components or to perform at increased power levels. An internal metal layer which is electrically insulated from the electrical components can dissipate such heat, but existing technology has not been satisfactory in producing such circuit boards. Electrophoretic deposition of the insulating layer typically is satisfactory for flat areas, but the insulation is too thin at sharp corners.
The typical circuit board has a number of holes through its body, generally termed via holes. These are occupied with conductive paths which connect circuitry on one surface of the board with circuitry on the opposite surface of the board. Each via has sides generally perpendicular to those opposing board surfaces. The sides of the hole, therefore, have corners which may be 90 degree angles, or somewhat less if they are mechanically rounded. These corners are those which typically coat too thin during electrophoretic deposition. This invention employs a thixotropic agent in the electrophoretic deposition solution to realize adequate coating, including at the edges of via holes. The thixotropic agent may also function to facilitate adhering metal to the outside of the metal surface as part of the circuitry.
Circuit boards with internal metal layers of the general kind of interest are described in the following: U.S. Pat. No. 4,321,290 to Thams; "Metal Core Boards--an alternative for cooling critical components," an article in "Circuits Manufacturing" September 1980, at pp. 74-80; and "Insulated Holes in Metal Core Substrates" by Keith F. Blurton, Francis J. Nuzzi and James Arachtingi, PCK Technology Division, Kollmorgen Corporation. All three of these disclose in more or less detail electrophoretic deposition to achieve an insulating layer on the internal metal body. None include a thixotropic agent in such an electrophoretic deposition liquid deposition composition.
Additionally, U.S. Pat. No. 4,145,460 to Finley et al is of general interest in that it teaches a printed circuit board with holes of the kind of interest with respect to this invention with a coating being from a powder. U.S. Pat. No. 3,892,646 to Lazzarini et al is of general interest in that it teaches electrophoretic deposition of insulating polymers of the kind of interest with respect to this invention with the coating not being for the internal metal of a circuit board and the coating bath not having a thixotropic agent.
U.S. Pat. No. 4,562,119 to Darms et al is of general interest in that it is directed to printed circuits and employs metal coated with polymers having siloxane groups. This invention differs basically in that it employs silicone dioxide or other thixotropic agent mixed with polymers.